The invention relates to an apparatus for producing low-birefringent and/or low stress plastic film or sheet having a high surface-polish and is suitable for optical media applications as well as low stress film for non optical applications either one using a continuous extrusion process. Optical media applications include such items as compact discs (CD), digital video discs (DVD), liquid crystal displays (LCD) or any other optical media applications which require a transparent substrate with low birefringence low stress and a high surface-polish. Non optical applications using low stress film or sheet for use in such applications as automobile dash board overlays or other uses for opaque film or sheet which require tight graphics registration. Birefringence is not measurable in opaque film or sheet.
More particularly, this invention relates to a particular calendering or process finishing roll stack wherein the structure of at least one of the finishing rolls is comprised of an inner steel shell, a resilient covering thereover and a multi layer metal sleeve outer covering. Film or sheet produced using the roll structure of this invention has low-birefringence, low stress and is highly polished on at least one surface i.e. a surface having a low roughness of 4 microinches or less which film or sheet is suitable for optical media applications or opaque film or sheet for such other non optical applications. Such film or sheet is produced in a one step continuous extrusion process.
Currently, polycarbonate is used as the polymeric material for optical media applications such as CD""s and are made by injecting molding. The process is relatively slow and expensive. In addition, it is difficult to produce CD""s of very low-birefringence which will be required to reach higher data densities in the future. CD""s currently produced today have a retardation value of 25-30 nm. (nanometers), which is birefringence times thickness. Stress and birefringence are inherent in injection molding CD""s because the melt starts to solidify on the inside mold wall as the mold is filling, and then additional melt is forced into the mold cavity to compensate for shrinkage of the disc as it solidifies. In opaque film or sheet, birefringence is not measurable but low stress is wanted for applications in vehicles, computer housings, etc. that require tight (0.4 mm/MAX) graphics registration.
Birefringence is defined as the difference between the refractive indices along two perpendicular directions as measured with polarized light along these directions. It results from molecular orientation, and the measurement of birefringence is the most common method of characterising polymer orientation. It is determined by measurement of the retardation distance by either a compensation or a transmission method. Positive birefringence results when the principal optic axis lies along the chain; negative birefringence when transverse to the chain. In Cartesian coordinates there are three birefringences, two being independent. Thus xcex94xy=nxxe2x88x92ny, the differences in refractive indices along the x and y axes. Uniaxial orientation only requires one of these to describe the orientation. Therefore, in order to obtain a uniform homogeneous polycarbonate, the lower the birefringence (the differences between the refractive indices) the more homogeneous the polymer composition of the product and thus the more uniform properties of the product. This is critical, particular on CD""s, DVD""s or LCD wherein the Laser read out must have minimal or zero distortion. The lower birefringence, the less is the variation in polymer homogeninity and Laser distortion.
Another parameter for optical materials is Cg which is the stress-optical coefficient of material in the glassy state. It can be measured with a molded part such as a small bar or disc. Birefringence can be measured by the method described above. When a stress is applied to the bar, the birefringence will change by an amount B. The stress-optical coefficient, which has units of Brewsters, is given by:
B=Cg xcex4
The stress-optical coefficient (Cg) should be less than or equal to about 70 Brewsters.
Improvements in optical data storage media, including increased data storage density, are highly desirable, and achievement of such improvements is expected to improve well established and new computer technology such as read only (ROM), write once, rewritable, digital versatile and magneto-optical (MO) disks.
In the case of CD ROM technology, the information to be read is imprinted directly into a moldable, transparent plastic material, such as bisphenol A (BPA) polycarbonate. The information is stored in the form of shallow pits embossed in a polymer surface. The surface is coated with a reflective metallic film, and the digital information, represented by the position and length of the pits, is read optically with a focused low power (5 mW) laser beam. The user can only extract information (digital data) from the disk without changing or adding any data. Thus, it is possible to xe2x80x9creadxe2x80x9d but not to xe2x80x9cwritexe2x80x9d or xe2x80x9cerasexe2x80x9d information.
The operating principle is a write once read many (WORM) drive is to use a focused laser beam (20-40 mW) to make a permanent mark on a thin film on a disk. The information is then read out as a change in the optical properties of the disk, e.g., reflectivity or absorbance. These changes can take various forms: xe2x80x9chole burningxe2x80x9d is the removal of material, typically a thin film of tellurium, by evaporation, melting or spalling (sometimes referred to as laser ablation); bubble or pit formation involves deformation of the surface, usually of a polymer overcoat of a metal reflector.
Although the CD-ROM and WORM formats have been successfully developed and are well suited for particular applications, the computer industry is focusing on erasable media for optical storage (EODs). There are two types of EODs: phase change (PC) and magneto-optic (MO).
Generally, amorphous materials are used for MO storage and have a distinct advantage in MO storage as they do not suffer from xe2x80x9cgrain noisexe2x80x9d, spurious variations in the plane of polarization of reflected light caused by randomness in the orientation of grains in a polycrystalline film. Bits are written by heating above the Curie point, TC, and cooling in the presence of a magnetic field, a process known as thermomagnetic writing. In the phase-change material, information is stored in regions that are different phases, typically amorphous and crystalline. The film is initially crystallized by heating it above the crystallization temperature. In most of these materials, the crystallization temperature is close to the glass transition temperature. When the film is heated with a short, high power focused laser pulse, the film can be melted and quenched to the amorphous state. The amorphized spot can represent a digital xe2x80x9c1xe2x80x9d or a bit of information. The information is read by scanning it with the same laser, set at a lower power, and monitoring the reflectivity.
In the case of WORM and EOD technology, the recording layer is separated from the environment by a transparent, non-interfering shielding layer. Materials selected for such xe2x80x9cread throughxe2x80x9d optical data storage applications must have outstanding physical properties, such as moldability, ductility, a level of robustness compatible with particular use, resistance to deformation when exposed to high heat or high humidity, either alone or in combination. The materials should also interfere minimally with the passage of laser light through the medium when information is being retrieved from or added to the storage device.
As data storage densities are increased in optical data storage media to accommodate newer technologies, such as DVD and higher density data disks for short or long term data archives, the design requirements for the transparent plastic component of the optical data storage devices have become increasingly stringent. Materials displaying lower birefringence at current, and in the future progressively shorter xe2x80x9creading and writingxe2x80x9d wavelengths have been the object of intense efforts in the field of optical data storage devices.
Birefringence in an article molded from polymeric material is related to orientation and deformation of its constituent polymer chains. Birefringence has several sources, including the structure and physical properties of the polymer material, the degree of molecular orientation in the polymer material and thermal stresses in the processed polymer material. For example, the birefringence of a molded optical article is determined, in part, by the molecular structure of its constituent polymer and the processing conditions, such as the forces applied during mold filling and cooling, used in its fabrication which can create thermal stresses and orientation of the polymer chains.
The observed birefringence of a disk is therefore determined by the molecular structure, which determines the intrinsic birefringence, and the processing conditions, which can create thermal stresses and orientation of the polymer chains. Specifically, the observed birefringence is typically a function of the intrinsic birefringence and the birefringence introduced upon molding articles, such as optical disks. The observed birefringence of an optical disk is typically quantified using a measurement termed xe2x80x9cin-plane birefringencexe2x80x9d or IBR, which is described more fully below.
For a molded optical disk, the IBR is defined as:
IBR=(nrxe2x88x92nxcex8)d=xcex94nrxcex8dxe2x80x83xe2x80x83(3) 
where nr and nxcex8 are the refractive indices along the r and xcex8 cylindrical axes of the disk; nr is the index of refraction seen by a light beam polarized along the radial direction, and nxcex8 is the index of refraction for light polarized azimuthally to the plane of the disk. The thickness of the disk is given by d. The IBR governs the defocusing margin, and reduction of IBR will lead to the alleviation of problems which are not correctable mechanically. IBR is a property of the finished optical disk. It is formally called a xe2x80x9cretardationxe2x80x9d and has units of nanometers.
In applications requiring higher storage density, such as DVD recordable and rewritable material, the properties of low birefringence and low water absorption in the polymer material from which the optical article is fabricated become even more critical. In order to achieve higher data storage density, low birefringence is necessary so as to minimally interfere with the laser beam as it passes through the optical article, for example a compact disk.
Materials for DVD recordable and rewritable material require low in-plane birefringence, in particular preferably less than about +/xe2x88x9240 nm single pass; excellent replication of the grooved structure, in particular greater than about 90% of stamper; and reduced water uptake as compared to BPA polycarbonate.
Another critical property needed for high data storage density applications, in particular DVD recordable and rewritable material, is disk flatness. The disk flatness is dependent upon the flatness of the polycarbonate substrate immediately after the injection molding process as well as the dimensional stability of the substrate upon exposure to high humidity environments. It is known that excessive moisture absorption results in disk skewing which in turn leads to reduced reliability. Since the bulk of the disk is comprised of the polymer material, the flatness of the disk depends on the low water solubility and low rate of water diffusion into the polymeric material. In addition, the polymer should be easily processed in order to product high quality disks through injection molding.
There is a distinct economic advantage of producing said film and sheet for discs for optical media applications via a continuous film extrusion process, whereby a continuous plastic web of 4-8 feet wide could be produced at speeds of 10-60 feet/minute from which discs could be cut out. Extrusion casting, where a melt is extruded through a slot die and deposited on a polished metal roller to solidify, can produce low-birefringence films but the top surface of the film is not smooth enough. Extrusion calendering, on the other hand, whereby a second polished metal roll is added to form a nip between the two rolls to squeeze the plastic on both sides as is solidifies, is widely used to produce very uniform and smooth-surface films. However, the flow in the nip between rigid rolls induces very high stresses and such films have retardation values of hundreds to thousands of nanometers. A resilient elastomeric cover has been put on one of the rolls to produce textured films that have lower stress, but the texture is unacceptable for optical media applications.
U.S. Pat. No. 3,756,760 teaches the use of a single metal outer sleeve of nickel over a rubber-covered roller to accommodate and smooth the non-uniformity of the extrudate from an extrusion die upon delivering melt to the calendering nip. It does not disclose how to use this to control stress in the film and birefringence. In addition, such a sleeve is too fragile to be of practical use.
U.S. Pat. No. 5,076,987 discloses producing optical quality extrusion film by calendering the film between a ground elastic roller and a high gloss steel roller to produce a film having a high gloss surface and a matte surface, or producing a film having a high gloss on both surfaces, by coating the matte surface.
U.S. Pat. No. 5,149,481 discloses extruding a sheet or film into the roll gap of a smoothed upper roll and a lower roll wherein the temperature of the upper roll is below the glass transition temperature of the plastic and the lower roll is maintained at a temperature in the plastic state domain of the plastic sheet or film.
U.S. Pat. No. 5,242,742 is similar to U.S. Pat. No. 5,149,481 except that it discloses a sheet or film having a birefringence of less than 50 nm and preferably less than 20 nm.
U.S. Pat. No. 4,925,379 discloses a process for producing a plastic sheet, wherein at least one layer is a polyurethane layer, by extrusion and pressing at a temperature higher than the softening point of the polyurethane.
U.S. Pat. No. 5,286,436 is a division of U.S. Pat. No. 5,242,742 and claims a sheet or film having a birefringence equal to or less than 50 nm, a low surface roughness and low variation in thickness.
All of the above references do not disclose or teach the particular finishing roll of the instant invention or a low birefringence, low stress highly polished film or sheet by a continuous extrusion process.
Accordingly, it is an object of this invention to produce a low birefringence, low stress, highly surface polished thermoplastic film or sheet on at least one surface thereof.
Another object of this invention is to provide means for producing a low birefringence, low stress, highly surface polished thermoplastic film or sheet on at least one surface thereof.
Still another object of this invention is provide a low birefringence transparent film or sheet suitable for optical media applications.
Yet, another object of the invention is to provide a one step continuous extrusion process for producing a low birefringence, low stress, highly surface polished transparent thermoplastic film or sheet.
These and other objects will become apparent from the following description of this invention.
The present invention is directed to products, apparatus and process for preparing thermoplastic film or sheet for optical media applications or non optical applications in the case of opaque film or sheet. The apparatus for producing a low birefringence, highly surface polished thermoplastic film or sheet comprises a calendering roll stack wherein at least one roll being of a particular novel construction in order to produce the product of this invention as well as a one step continuous extrusion process for producing the product of this invention which product is a low birefringence, low stress, highly polished transparent film or sheet or a low stress highly polished opaque film or sheet.
It has been surprisingly discovered that the novel structure of the calendering roll of this invention comprises at least a three component calendering roll structure comprised of an inner metal shell, an intermediate resilient elastomeric cover over the inner metal shell and a multi-layer metal sleeve outer covering. The multi-layer metal sleeve outer covering comprises at least two layers and preferably a three layer metal sleeve outer covering. The novel calendering roll of this invention and an opposing calendering roll form a calendering nip or gap through which thermoplastic film or sheet is continuously extruded. This process is also known as a continuous film or sheet extrusion process. As used herein, the terms xe2x80x9cfilmxe2x80x9d and xe2x80x9csheetxe2x80x9d are used interchangeably and refer to thermoplastic material having a final thickness of about 0.001 to about 0.060 inches but may be thicker, if so desired, depending on the final application.